1. Field of the Invention
The present invention relates to a control station apparatus and a control method thereof, a communication apparatus and a control method thereof, and a wireless communication system.
2. Description of the Related Art
As a conventional scheme that transmits data in a stable manner between devices connected via a communication path, a scheme is known in which plural wireless stations that relay data on transmission paths between devices are disposed, and relay transmission paths are realized over plural paths (for example, see Japanese Patent Laid-Open No. 2005-236632). Additionally, a scheme that causes a wireless terminal station to operate as a relay station rather than using a dedicated relay station is known as a scheme for carrying out relay transmission (for example, see Japanese Patent Laid-Open No. 2003-332977).
Furthermore, a scheme that transmits the same data plural times in order to realize an improvement in the reliability of the data to be transmitted through redundant data transmission is known (for example, see Japanese Patent Laid-Open No. 2000-22671). In particular, as a method for transmitting data, a scheme that controls plural transmissions of broadcast or multicast data to which retransmission control cannot be applied is known (for example, see Japanese Patent Laid-Open No. 2003-37606).
With the method that configures plural paths using the conventional relay transmission path, the state of traffic on the transmission path and the connection status of the line are confirmed, and in the case where a problem has arisen in the transmission path, the data transmission is carried out on plural pre-set paths (Japanese Patent Laid-Open No. 2005-236632). However, in this configuration, the transmission path is switched after the occurrence of a problem on the transmission path has been detected. For this reason, there is a problem that when transmitting stream data, which has a short period of validity (lifetime), communication is momentarily interrupted, and transmission cannot be carried out correctly. There is another problem that when performing transmission of stream data synchronized with plural devices from a data transmitting device, different transmission paths are set for each device, and thus the time at which the data will reach the devices cannot be guaranteed.
Furthermore, there is a method in which a terminal station connected to a control station detects other terminal stations with which communication is possible; after this, a representative relay terminal is determined and a terminal group configured, whereby data is transmitted and relayed from the control station to terminals under the control of the representative relay terminal (Japanese Patent Laid-Open No. 2003-332977). This method also proposes causing the frequencies used by the control station and the terminal group to match, whereby the terminal station that correctly receives data relays the data to the representative relay terminal in the case where the representative relay terminal could not correctly receive the data. However, in this configuration, the data transmitted to the terminal station under control of the terminal group terminates once at the representative relay terminal, whereupon the representative relay terminal performs transfer control that includes retransmission. For this reason, in the case where retransmission control is undertaken between the control station and the representative relay terminal, a significant delay occurs in the transmission of data between the control station and the terminal. There is another problem that a procedure is necessary for the terminal station to be aware that the representative relay terminal has failed in receiving the data, in the case where the terminal station carried out relays; accordingly, an excess control procedure arises in addition to the normal communication process, and the transmission delay increases even more. There is yet another problem that a procedure for switching retransmission control among terminal stations with which the control station does not communicate directly is also necessary, and thus the control becomes complicated.
Within the redundant data transmission method, there is a technique in which the same packet is transmitted multiple times, the received packets are compared, and the optimum packet is selected as a result of a majority decision (Japanese Patent Laid-Open No. 2000-22671). There is also a method that suppresses needless transmission by controlling the number of transmissions in accordance with the status of the system (Japanese Patent Laid-Open No. 2003-37606). However, there is a problem with this method in that the effects of this method cannot be obtained when the wireless transmission path is temporarily unable to carry out communication, in the case where the same packet is transmitted plural times between terminals.